This invention relates generally to a mass flowmeter for measuring the mass flow of a fluid and, in particular, to a mass flowmeter suitable for measuring the air intake flow of an internal combustion engine.
Karman vortex type flowmeters are known as prior art flowmeters for measuring the air intake flow of an internal combustion engine. This type of flowmeter has a pillar-shaped bluff body which is placed at right angles in the flow for producing a Karman vortex street. The shedding frequency of this Karman vortex street is proportional to the flow velocity of the fluid so, by detecting the vortex street shedding frequency, it is possible to determine the flow velocity, and from this flow velocity it is possible to measure the flow rate.
This kind of Karman vortex type flowmeter has a simple structure and the resistance offered by the bluff body to the flow is low so that this type of flowmeter is very suitable for measuring the flow rate of the air intake of an internal combustion engine.
As is clear from this description, the Karman vortex type flowmeter measures the volumetric flow. However, when measuring the air intake flow of an internal combustion engine, it is desirable to measure mass flow rather than the volumetric flow. The reason for this is that in order to accurately control the air-fuel ratio, it is necessary to detect the mass flow of the intake air to eliminate the effects of changes in intake air pressure and temperature.
Japanese Patent Disclosure No. 51311/80 discloses an intake air detector which is an improvement on the Karman vortex type flowmeter. This intake air detector obtains the mass flow of the fluid by correcting, in response to the density of the fluid, the volumetric flow obtained by the Karman vortex type flowmeter. To elaborate, the prior art intake air flow detector changes the cross-sectional area of the intake pipe and varies the speed of the intake air flow inside the intake pipe in response to the density of the intake air. This kind of a device not only requires means for varying the passage cross-sectional area but also requires sensor means for detecting the density of the intake air, resulting in a complicated and large structure.
U.S. Pat. No. 4,116,060 also discloses a mass flowmeter, which is an improvement on the Karman vortex type flowmeter. This mass flowmeter is different from the intake air flow detector disclosed above. The characteristic dimension of the bluff body can be varied in response to changes in the air density to thereby obtain a value of the Karman vortex shedding frequency which is proportional to the mass flow the air intake. However, with the the mass flowmeter of this U.S. Patent, in making the characteristic dimension of the bluff body variable, the shape of the body also varies greatly so there is a change in the Strouhal number, which affects the Karman vortex shedding frequency. Consequently, the shedding frequency is not accurately proportional to mass flow of the fluid making accurate detection of the mass flow impossible.